1. Field of the Invention
The present invention relates to a new aqueous coating composition based on organic polyhydroxyl compounds and organic polyisocyanates containing free isocyanate groups and to the use of this coating composition for coating mineral substrates.
2. Background of the Invention
Reducing the organic solvent content of lacquers and coating compositions is a current problem for ecological and industrial hygiene reasons. Accordingly, the replacement of organic solvents by water is of considerable importance in modern coating technology.
Known aqueous two-component polyurethane lacquers contain blocked polyisocyanates because it has previously been assumed that polyisocyanates containing free isocyanate groups would not be useable in lacquer systems of the type in question because it would be expected that the polyisocyanates would react with the large excess of water and would not selectively react with the organic polyhydroxyl compounds that are always present as the main binder component of such coating compositions.
Aqueous lacquers based on blocked polyisocyanates are described, for example, in DE-OS 2,507,842 or 2,507,884. The systems according to DE-OS 3,829,587 are an exception to the rule mentioned. The aqueous lacquers described in this publication are in fact lacquers which, in addition to organic polyhydroxyl compounds, contain polyisocyanates containing free isocyanate groups. The selective reaction of the polyisocyanates with the organic polyhydroxyl compounds in the presence of a large excess of water is attributable to the very special nature of these polyhydroxyl compounds.
A completely new aqueous coating composition based on organic polyisocyanates and organic polyhydroxyl compounds has now surprisingly been found, containing water-soluble polyether polyols, nonionically hydrophilically modified polyisocyanates and magnesium oxide or hydroxide as its principal binder components. The coating composition according to the invention, which is described in more detail hereinafter, is particularly suitable for coating mineral substrates, such as masonry, concrete, gypsum and the like.